1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method and, more particularly, to an image processing apparatus and image processing method, which correct chromatic aberrations due to optical factors and suppress any color fringing.
2. Description of the Related Art
Recent digital video cameras and digital cameras can obtain high-quality images by adopting image sensors with a large number of pixels.
On the negative side, the influences of chromatic aberrations resulting from different focal lengths for respective light wavelengths and, more specifically, color fringing readily appear in images due to factors of miniaturized pixels, compact lenses, and so forth.
Conventionally, as a technique for suppressing any color fringing in a sensed image, various methods have been proposed. Japanese Patent Laid-Open No. 2007-195122 proposes a technique for suppressing any color fringing by detecting an edge in an image acquired by an imaging device, and reducing a tone of a color signal near the detected edge position according to tone reduction rules obtained based on optical parameters.
For example, when a high-power zoom lens, which is often used in video cameras, is used, the color fringing characteristics vary intricately depending on the image height from the optical axis center to a pixel of interest, zoom ratio, aperture value, and focus lens position. For this reason, the simple tone reduction rules described in Japanese Patent Laid-Open No. 2007-195122 cannot effectively suppress any color fringing. Therefore, in practice it is difficult to control the color fringing suppression level using only the position of the pixel of interest within a frame as a criterion.
The method described in Japanese Patent Laid-Open No. 2007-195122 cannot attain color fringing suppression for RGB signals. For this reason, this method must be mere achromatic processing that adjusts the gains of color difference signals, and is not suitable for color fringing suppression processing.
Japanese Patent Laid-Open No. 2003-102027 and Japanese Patent Laid-Open No. 2003-102028 disclose the following technique. That is, color fringing is determined by seeing if a degree of change in color difference data and a difference between colors exceed corresponding thresholds. Also, edges are detected from image data, and color fringing is suppressed using a low-pass filter (LPF) and median filter depending on positions near the edges.
However, since color fringing region determination is binary determination processing as to whether or not to apply color fringing suppression, when low thresholds are set for the degree of change in color difference data and the difference between colors, a color edge part of an image signal having a strong gradient is erroneously determined as a color fringing part. As a result, the color edge part where no color fringing occurs is erroneously corrected, in other words, correction of color fringing occurs where it is not required as well as where it is required.
By contrast, when high thresholds are set for the degree of change in color difference data or the difference between colors so as to avoid such determination errors, color fringing that occurs in a low-gradient part cannot be determined, and color fringing correction becomes insufficient, in other words, correction of color fringing is not always performed when required. Also, since color fringing corrected part and non-corrected part are clearly differentiated, corrected and non-corrected parts in one image may be visualized as steps, or those between frames of a movie may be perceived as steps.